Impedance matching by means of coupled helices



Sept. 22, 1959 IMPEDANCE MATCHING BY MEANS OF COUPLED HELICES Filed June30. 1953 c. c. C UTVLER 2,905,858

PHASE DELAY \-/PHASE CURVE a FREQUENCY INVENTOR C. C. CUTLER ATTORNEYSept. 22, 1959 c c, T R 2,905,858

IMPEDANCE MATCHING BY MEANS OF COUPLED HELICES Filed June 30. 1953 2Sheets-Sheet 2 INVENTOR C C. CUTLER ATTORNEY United States IMPEDANCEMATCHEQG BY MEANS OF COUPLED HELICES Application June 30, 1953, SerialNo. 365,210

16 Claims. (Cl. SIS-39.3)

This invention relates to impedance matching and more particularly itrelates to impedance matching between circuits having greatly differingimpedances.

An object of this invention is to provide an improved impedance matchingarrangement for use in traveling wave tubes.

One common form of traveling wave tube employs an electron stream incombination with a concentric wave propagating helix which is adapted topropagate an electro-magnetic Wave at substantially the same velocity asthat of the electron stream. When a signal wave travels along this helixin synchronism with the stream it is able to extract kinetic energy fromthe electrons and thereby to increase its own energy. It has been found,however, that under certain conditions this kind of amplification isunstable because of the presence of waves backward traveling withrespect to the electron stream which are of 'sufficient amplitude tosustain oscillations. Since such oscillations lower tube efficiency andproduce other unwanted results, it is desirable therefore to preventthem by, for example, reducing below a critical value the feedbackenergy supplied by the backward traveling waves. To this end it is veryhelpful to eliminate so far as possible any impedance mismatches betweeninput or output circuits and the helix circuit in the tube since thesemismatches would otherwise cause wave reflections. Moreover, frequentlythis matching is made more .difficult than it otherwise would be by thelarge difference between the impedance of conveniently available inputor output circuits and the impedance of the tube helix.

Previous to this invention, one way of matching these impedances hasbeen to place a shorting bar or its equivalent across several turns ofthe tube helix near the end thereof thereby forming a T .or 1r typefilter between the helix and an input or output line. Such a filter hasimpedance transforming properties but unfortunately the frequency bandWidth over which such action is efficient rangement whereby asubstantially reflectionless match over an acceptable band width can beobtained between circuits having greatly differing impedances.

In accordance with the present invention and in one embodiment thereof aribbon helix of low impedance is asymmetrically placed in couplingrelation around the wave propagating helix in a traveling wave tube. Thepitch of the outer helix is made substantially greater than the pitch ofthe inner helix and by this means its impedance is made appreciablysmaller than that of the inner helix.

Impedance matching between the two helices is accomplished simply byadjusting the pitch .of the outer helix so that a component of a wavepresent on it travels therealong at the same axial phase velocity asthat of a wave on the inner helix. This synchronization of phasevelocity takes place by spatial harmonic interaction whereby the wave onthe outer helix traveling at greater axial phase velocity than that of awave on the inner helix interacts with the latter wave only at givenintervals so that it-appears to be moving in synchronism with it.

A more complete understanding of this invention, together with a betterappreciation of its advantages will best be gained from a study of thefollowing description given in connection with the accompanyingdrawings, in which:

Fig. l is a schematic representation of the input portion of a helicaltraveling wave tube in which the tube amplifying helix is coupled at anend thereof to an outer ribbon helix;

Fig. 1A is a cross section of the helices in Fig. 1 taken as indicatedby line 1A1A;

Fig. 2 is a plot of the phase constant versus frequency characteristicsfor two helices such as shown in Fig. 1;

Fig. 3 shows an arrangement similar to that in Fig. l in which the outerhelix is wire wound with non-uniform pitch;

Fig. 4 shows an arrangement similar in operation to that in Fig. l inwhich a filter type spatial harmonic circuit is coupled to the innertube amplifying helix; and

Fig. 5 shows a modification of the embodiment of Fig. 1 in whichcoupling to the inner helix. takes place by means of an intermediatehelix inserted between it and the outer helix.

Referring now more particularly to .the drawings, Fig. 1 shows by way ofillustration a side section of part of a helical traveling wave tube 10in which helix 11 is the. principal wave propagating circuit. Electrongun 12, positioned to the left of this helix, is aligned with respectthereto so that electron stream 13 flows axially within helix .11.Ribbon helix 14 surrounding the left end portion of this helix isseparated therefrom by insulating material 15 which is preferably alow-loss dielectric material such as quartz. The rightend of helix 14 isterminated in a nonreflecting impedance 16 which may be placed aroundthe last several turns as shown. The other end of helix 14 is connectedto and forms a continuation of the inner conductor of coaxial input line17.. The outer conductor of this line forms an over-all envelope for thetube and extends on beyond the output end of the tube, which, althoughnot shown, may be similar to the input end. Power supply 18 foraccelerating electron stream 13 and cathode heater battery 19 may beconnected as shown in Fig. 1. Other auxiliary circuits not shew-n herecan be substantially the same as those customarily provided forconventional traveling wave tubes and their use is to be understood.

It will be noted from the drawing that the pitch of helix 14 is oppositewith respect to and much larger thanl'the pitch of helix 11. Because ofthis, its impedance is much lower than that of the helix it surroundsand its length is ppreciably shorter than would be necessary if it werewound in the same sense as the inner helix. The lengths of helix l4 andof the other spatial harmonic circuits illustrated herein depend uponthe tightness .of coupling and on the order of the mode of operation. Ingeneral these lengths should be sufficient to insuresubstantiallycomplete energy transfer.

In the absence of asymmetry in the physical relation of inner to outerhelices the useful field components of the wave on the outer helix wouldbe self-canceling in their e'ifect on'the inner helix. Thisrequirement-of asymmetry 2.905.858 i u i is easily met, however, byproviding, for example, a flat side on helix 14 as is shown in theenlarged cross section thereof shown in Fig. 1A taken as indicated byline 1A-1A in Fig. l. The inner flat surface of the outer helix is woundin close proximity to the outer surface of helix 11 so that the fieldcoupling between the two circuits will be large. The coupling betweenthe two circuits is best described as being periodically variable alongthe coextensive lengths of the two circuits. Thus the fiat side on helix14 is a region of strong coupling which occurs periodically along thelength of helix 14.

The operation of the embodiment in Fig. 1 will per haps best beunderstood from a consideration of Fig. 2. This figure shows by way ofexample the phase delay ,8 versus frequency to characteristics for twohelices such as those shown in Fig. 1. Curve A, which may be taken torepresent the characteristic of helix 11, is approximately a straightline through the origin for the portion of the characteristic shown.This linearity results from the essentially constant phase velocity ofpropagation, which for any circuit is given by of a wave traveling alonga helix whose dimensions are small relative to a wavelength. Curve B,representing the characteristic of helix 14. shows by the solid portionof the curve for values of 6 below the value F that the axial phasevelocity of the fundamental component of a wave traveling along thishelix is roughly constant with frequency and is much higher than thevelocity of a wave along helix 11 over this range of frequency. No synchronization is possible therefore between this fundamental componentand a wave on helix 11. It is evident, however, from an inspection ofFig. 2 that at certain frequencies, fixed by the intersections of curvesA and B, the velocities of wave components on the two helices are thesame. The first of these intersections, point P, represents zerovelocity dilference between the first backward traveling spatialharmonic components on helix 14 and the main Wave on helix 11. This kindof synchronization is useful when the signal wave on helix 11 is backward traveling with respect to the direction of electron flow. For thestructure illustrated in Fig. 1, however, where the signal is forwardtraveling synchronization with forward traveling components on helix 14is necessary. The first of such points of synchronization occurs at intersection I in Fig. 2 where the first forward spatial harmonic on helix14 will travel with substantially the same velocity as that of thesignal wave on the inner helix. When curve A is tangent to curve B asshown there is an appreciable band width of frequency over which the twovelocities are equal. Although higher order modes of synchronization,both backward and forward, are possible, in general it is desirable toutilize a low order component of the fundamental wave because as theorder increases the amplitude decreases. Accordingly the pitch of helix14 is preferably chosen so that the first spatial harmonic wave on itwill synchronize with the wave on the inner helix.

The magnitude of the slope of the phase delay versus frequencycharacteristic of a circuit is proportional to its impedance and so itcan be seen in Fig. 2 from curves A and B in the vicinity ofintersection I that the impedance of helix 14 is much lower than that ofhelix 11. Therefore, by properly adjusting the pitch and the phase delaycharacteristic of the outer helix it is readily apparent that a lowimpedance line can be matched to a high impedance circuit.

In obtaining a thorough understanding of the invention it is helpful toconsider from a mathematical standpoint the relationship between thefundamental component of the wave on helix 14 and its spatial harmoniccomponents. Let (p be the phase displacement in radians between adjacentturns and let a! be the center to center distance between turns. Thenthe axial component ll, of the electric field of the wave travelingtherealong is given by the following expression:

where '11: 2 F(z)= 2 '(j +)a Thus E is represented as an infinite sum oftraveling waves each with a difierent phase velocity v given by and 21m+(2) where n is an integer between +00 and oc. By adjusting the pitch anddiameter of helix 14 so that the ratio is equal to the axial phase delayconstant B of the inner helix for a given value of 21, substantiallycomplete transfer of energy between the two helices can be obtained;

Fig. 3 shows a second illustrative embodiment of the invention in whicha traveling wave tube helix 31 is surrounded by round wire helix 32having a nonuniform pitch. This outer helix, because of its variablepitch, functions in substantially the same way as does helix 14 inFig. 1. Here, however, instead of forming a continuation of a coaxialline surroundingother tube elements, its left-hand end may be connectedto a line 33 as shown. A matched impedance 34 is connected to the otherend of helix 32 in order to eliminate reflections of wave energy at thispoint. The left-hand end of helix 31 may also be so terminated althoughthis is not shown in the drawing. Glass insulation 35 serves both as ameans of separating the two helices and as an air-tight envelopesurrounding electron stream 36.

Fig. 4 shows a third possible embodiment of the invention in which afilter type spatial harmonic circuit is placed surrounding a travelingwave tube helix 41. This filter circuit consists of a succession ofrings alternately connected in two groups, one of which (group 42) formsa continuation of the inner conductor of input coaxial cable 43. Bygrounding the other group (group 44) a periodic shielding of theelectric field in this region is produced, thereby causing thegeneration of spatial harmonic components of the principal wavetraveling along this section. The coaction of this circuit with thehelix it surrounds, as well as their remaining structural details aresubstantially the same as those of the circuit of Fig. 1.

Fig. 5 shows an illustrative embodiment of the invention in whichimpedance matching between an inner wave helix 51 of a traveling wavetube and an outer ribbon helix 52 takes place in two steps instead ofone as previously. This is accomplished by inserting an intermediateribbon helix 53 between the inner and outer circuits. To secure economyof space while at the same time insuring strong coupling, theintermediate helix should be wound in a sense opposite to that oftheother two helices and both it and the outer helix should beasymmetrically placed around the inner helix. Other structural details,of this arrangement, can be similar to those described previously inconnection with Fig. 1.

Since the impedances of coaxial line 54 and helix 52 are matched, thestanding wave ratio at their junction is substantially unity and all theenergy from the line flows into this helix. By adjusting the pitch ofintermediate helix 53 so that the velocity of a low order spatialharmonic component of a wave on it is substantially the same as thevelocity of a higher order component of the wave on the outer helix,energy transfer between the two helices takes place. Energy transferbetween helices 53 and'51 takes place in a way which has previously beenset forth. Thus by proper adjustment of all these factors, a doublestep-up of impedance line 54 to helix 51isobtained;

It is apparent from the embodiment of Fig. that a helix such as helix 53may be used instead of round ,wire inner helix 11 in'Fig. 1.Other'obvioussubstitutes for helix 11 include a filter type circuit suchas shown in Fig. 4 and a bifilar helix. In general,-for the purposes ofthis invention, any appropriate spatial harmonic circuit may be coupledto any other suitable circuit, spatial harmonic or otherwise, in a waysimilar to those set forth herein. Other changes or modifications of theembodiments illustrated-will occur to those skilled in the art and maybe made without departing from the spirit or scope of this invention. 7a

What is claimed is:

1. In combination, a first wave propagating circuit having a givenimpedance and adapted. to propagate a first electromagnetic wave alongan axiswith a given phase velocity,.a second wave propagating circuithaving an impedance different from said given impedance and adapted topropagate a second electromagnetic wave in a direction parallel to saidaxis with a phase velocity different from said given phase velocity,means for applying a wave signal to one of said circuits, said secondcircuit being insulated from said first circuit and positioned inelectromagnetic field coupling relation to said first circuit only atperiodic intervals determined so that said second wave appears to betraveling in the same direction as said first wave with substantiallythe same phase velocity, whereby wave energy is coupled between saidfirst and second circuits at said intervals.

2. The combination of elements as in claim 1 in which said first andsecond circuits are helices of first and second pitches respectively.

3. The combination of elements as in claim 1 in which said secondcircuit surrounds said first circuit and is shielded therefrom atperiodic intervals spaced along the axis of wave propagation.

4. A traveling wave tube including, a coaxial transmission line havingan outer conductor and an inner conductor a length of said innerconductor being removed along a section of said line, a first wavepropagating circuit adapted to propagate a spatial harmonic component ofa wave on said transmission line at a phase velocity v and forming acontinuation of said inner conductor along part of said section, asecond wave propagating circuit disposed along said section and adaptedto propagate a wave component of a wave at substantially the velocity v,said first circuit having a plurality of spaced portions along thelength thereof having a wave energy coupling characteristic with saidsecond circuit greater than the Wave energy coupling characteristic ofthe remaining portions of said first circuit with said second circuitwhereby wave energy is coupled between said first and second circuits atsaid portions, and means for forming and projecting an electron streamin field coupling relation to said second circuit.

5. The combination of elements as in claim 4 in which said first circuitis a ribbon helix.

6. The combination of elements as in claim 4 in which said secondcircuit is a wire helix having a common axis with said inner conductor,and said electron stream is projected along said axis.

7. In combination, a wave transmission helix having a phase delaycharacteristic of ,8 radians per unit length, and a wave propagatingcircuit surrounding a portion of the length of said transmission helixfor transferring wave energy between said helix and said circuit, saidcircuit having a plurality of spaced portions along the length thereofhaving a wave energy coupling characteristic with said helix greaterthan the wave energy coupling characteristic of the remaining portionsof said circuit with said helix, said circuit having a phase delaybetween turns of radians, a spacing between turns d and a diameter suchthat 6 Zrn-Ho is substantially equal to [3,where n is any integer otherthan zero whereby wave energy is coupled between said means andcoextensive therewith for a portion of its length and adapted topropagate a component of a wave having a frequency in along said givendirection with a velocity v, said first means having a plurality ofspaced portions along the length thereof coextensive with said secondmeans having a field coupling characteristic with said second meansgreater than the field coupling characteristic of the remaining portionsof said first means with said second means whereby wave energy iscoupled between said second means and said first means at said spacedportions.

9. In combination, impedance matching means including a plurality ofcoextensive wave propagating circuits each having a different impedanceand each placed adjacent to and insulated from another of said circuitsfor transferring wave energy between adjacent circuits, a first one ofsaid circuits adapted so that at a given frequency the phase delay inradians per unit length of a spatial harmonic component of a wavetraveling therealong in a given direction is substantially equal to thephase delay in radians per unit length of a wave traveling along anadjacent circuit in a parallel direction, said first one of saidcircuits having a plurality of spaced portions along the length thereofcoextensive with said adjacent circuit having a wave energy couplingcharacteristic with said adjacent circuit greater than the wave energycoupling characteristic of the remaining portions of said first one ofsaid circuits with said adjacent circuit whereby wave energy is coupledbetween said first one of said circuits and said adjacent circuit.

10. In combination, a source of a high frequency signal wave, atransmission line of given impedance connected to said source, a firstwave transmission circuit of matched impedance connected to said lineand adapted to propagate along a given direction with a phase velocity va spatial harmonic component of the wave thereon, and a second wavetransmission circuit of different impedance placed in proximity to saidfirst circuit and coextensive therewith over a portion of its length andadapted to propagate parallel to said given direction with substantiallythe velocity v a component of a wave having the same frequency as saidsignal wave, said first circuit having a plurality of spaced portionsalong the length thereof coextensive with said second circuit having awave energy coupling characteristic with said second circuit greaterthan the wave energy coupling characteristic of the remaining portionsof said first circuit with said second circuit whereby wave energy iscoupled between said first and second circuits at said spaced portions.

11. The combination of elements as in claim 9 in which the first one ofsaid circuits is a ribbon wire helix having a pitch d and adapted togive spatial harmonic wave propagation, and a second one of saidcircuits is a round wire helix having a pitch less than b and beingasymmetrically surrounded by said ribbon helix.

12. The combination of elements as in claim 9 in which the first one ofsaid circuits is a helix of low impedance adapted to support spatialharmonic wave propa gation, a second one of said circuits is a helix ofintermediate impedance adapted to support spatial harmonic wavepropagation, and a third one of said circuits is a helix of highimpedance whose dimensions are small enough so that it will not supportspatial harmonic wave propagation.

14,. The combination of elements as in claim 9 in which a first one ofsaid circuits is an iterative .filter type circuit adapted to supportspatial harmonic wane propagation, and asecond one of .said circuits isa wirewound helix of uniform pitch.

-15.. The combination of elements :as in claim Ali) in whichtsaid-firstcircuit is adapted to-propagate with velocity v the first forwardtraveling spatial harmonic component of the wave traveling .therealongsand saidsecond circuit, is. adapted to propagate with velocity v a wavenot having spatial harmonic. components.

116. The combination of elements as .in claim 10 in which said secondcircuit is adapted to propagate with velocity v the first forwardtraveling spatial harmonic component of the wave traveling therealongand said first circuit is adapted to propagate a spatial harmonic wave.20

UNITED STATES PATENTS Piley L "Feb. 5, Hansel! ;'.i. Mar. .11, -ri--;........ May 5., =Bieme,, May 3 Field Nov. 29,

Ettenberg July 3,

"Samuel -s Aug. 7, iNergaard...r Feb. .19, :Bryant.-- Aug. 20, Cutler"Oct. 1,

Peter Nov. 12,

FOREIGN 'PAIEENIS.

France Apr. 11,

Germany l.. Nov. 20,

